Recent information has shown that the development of the surfactant lining of the fetal lung entails both a qualitative as well as a quantitative change in the phospholipids that make up the surfactant. Development proceeds in "stepwise" fashion: the lecithin concentration increases; concurrently there is an increase in concentration of phosphatidyl inositol (PI), an acidic phospholipid associated with alveolar stability in the fetus; then the PI decreases when concomitantly phosphatidyl glycerol (PG) appears and increases. With the advent of PG, surfactant is stable and mature. These studies are delineating the mechanisms of biosynthesis on a subcellular basis and studying the effects of PG on stability and surface tension characteristics of surfactant, principally using rabbits as an experimental species. Corollary studies are being done in humans using amniotic fluid and tracheal aspirates after birth. These studies show that PG appears after 36-37 weeks gestation where upon the lung of the fetus is mature and no longer subject to RDS, even should asphyxia occur.